SCP-300x is contained in a heat-proof gas cylinder located in room number 278 on Site-66.

Part of this is related to special containment procedures, but the sentence is written as description. Generally, I feel that it's a better idea to wait until the description to drop the site your SCP is stored in, just so the misunderstanding that your SCP HAS to be stored at Site-66 is avoided.

The room must be airtight equipped with an entry lock that can be opened only by a level IV entry card.

The grammar on this sentence is a little bit wonky, but it can be fixed by adding "and" between airtight and equipped. The key-card bit sounds like it's been taken from Containment Breach. While key-cards are nifty, I would avoid writing it in this manner because it implies that only the key-card is necessary. Any personnel that is given, or steals, the appropriate key-card is technically allowed access according to your instructions. I would recommend instead using the clearance level of personnel, not cards, in restricting access.

Due to it's lethality, SCP-300x needs to be manipulated in a -10°C room and by at least 5 highly-trained scientists wearing Hazmat suits.

This is more of a personal gripe, but I don't like seeing the phrase "due to (lethality/cuteness/whatever)" preceding containment procedures. This is mostly because the Foundation naturally assumes that the special containment procedures are tailored to the dangers and attributes of the SCP. I would recommend just moving on to the instructions. "5 highly trained scientists" is likewise redundant since, even without it, readers will assume that the Foundation isn't sending coat-and-tie scientists who have never handled hazardous materials into this room.

You'll notice that I haven't really touched on the actual procedures themselves. This is because I'm conflicted. One on hand, I don't know how diseases are stored. On the other hand, this nerve agent, no matter how horrific, appears to behave like an ordinary chemical weapon. As a result, I would recommend supplanting your special containment procedures with:

SCP-300x is to be kept in a standard pathogenic/hazardous material containment unit with an internal temperature maintained at (insert numbers and degrees here)

This will suffice unless you add attributes to your SCP that make these instructions either useless or counter-active to the purpose of containment.

Extended breathing of SCP-300x will cause the melting of the Iris leaving the infected person with blank eyes and blindness.

Scientific tone could be tightened up here. "Blank eyes" isn't really a medical term, nor a detached observation. I would recommend rephrasing this to something along the lines of:

Prolonged exposure to SCP-300x will result in the victim's irises dissolving, leading to blindness and aniridia.

Please note that aniridia is the term for a lack of irises. However, this does not cause the blank eye effect you desire because you mean to refer to "Pupils."

After 5 minutes, the infected will experience theses symptoms:

Paranoia

Insanity

Skin bleaching

Shakiness

Profuse salivation

If not neutralized,the infected will ear highly pitched noises and try kill himself by smashing his head against walls or any ways possible.

This isn't very interesting. I'll be honest, it sounds as though the concept behind this SCP is "nerve agent that does disgusting things to people and then makes them kill themselves." I would recommend reconsidering the concept because an SCP usually needs more than just a danger level to be successful. There are real life chemical weapons and diseases more frightening than this and the brevity of this article, combined with the near total lack of addenda or embedded story, made me quickly lose interest.

Once neutralized, the corpse must be incinerated by a cleaning unit wearing anti-microbial suits and gas masks.

I take it this is meant to prevent the nerve agent from spreading. If so, this belongs in the special containment procedures.

Researchers believe that the only way to synthesize the nerve agent in a safe way is to do it one time per month with military grade equipment.

This read to me as unnecessary and rather unfinished. I was never really wondering how this SCP could be artificially reproduced, but I did begin to wonder why it's only safe to reproduce once per month. This question was never answered. I would recommend expanding heavily on this part if it is important to your SCP. If you do that, I would also recommend leading up to this point in other parts of the article so that synthesis does not feel like it came out of nowhere.

This is problematic. You indicated earlier that the Foundation understood synthesis could occur so long as it was once per month. And yet, only one successful synthesis has occurred? This is not only kind of contradictory, but also somewhat unrealistic as the Foundation would rarely jump to conclusions based off data from a single test. I would recommend getting a solid idea of whether or not SCP-300x can be synthesized, and then making your writing match that understanding in a way that makes sense to both the reader, and the scientific method.

I don't think this was necessary. If anything, this only furthers the contradiction issue I highlighted above because now there's an unmentioned temperature variable required in synthesis, not just equipment or time frame ones. Additionally, it seems to just reaffirm the reason for the exact temperatures listed in the procedures. I would recommend using testing logs such as this to show off your SCP's capabilities and/or to apply your SCP in interesting ways.

In conclusion, this is a very bare-bones SCP. It's a nerve agent that does bad things to you and then gets you to kill yourself but that's it. I would recommend heading back to the brainstorming forums to figure out how to make the idea of a sarin-based nerve agent truly interesting, as well as how to play around with formatting and syntax to add in a story or highlight any interesting additions you make. Remember, this site has a lot, a LOT, of SCPs which are dangerous and nothing else. The days where those could pass have faded. Feedback over. Good luck!